Popular whim of early childhood? (7)
I believe the answer is:
infancy
'early childhood?' is the definition.
('infancy' can be a synonym of 'early childhood')
'popular whim' is the wordplay.
'popular' becomes 'in'.
'whim' becomes 'fancy' (I've seen this before).
'in'+'fancy'='INFANCY'
'of' is the link.
(Other definitions for infancy that I've seen before include "Babyhood" , "Early years" , "State of early childhood" , "youth" , "Early stage of development" .)